JS: Function.prototype.apply

f.apply(obj) → evaluate f() with f's this keyword having value of obj.

f.apply(obj, array) → evaluate function f with f's this keyword have value of obj, and elements of the array array is passed to the function as arguments. array can be array-like object.

// example of using f.apply
function ff() { return this; }
const x2 = {kk:373};
console.log ( ff.apply( x2 ) ); // { kk: 373 }

Example with 2 arguments:

// example of using apply

function ff(a,b) {
    this.x = a;
    this.y = b;
}

const x3 = {};

ff.apply(x3, [7,8]);

console.log( x3 ); // { x: 7, y: 8 }

Second arg of apply can be array-like object.

function ff(a,b) {
    this.x = a;
    this.y = b;
}

const x3 = {};

// 2nd arg of apply can be array-like object
ff.apply(x3, {0:7,1:8,length:2});

console.log( x3 ); // { x: 7, y: 8 }

The “apply” method can be used to get the max value of a array. See: JS: Get Max/Min Value of Array.

Reflect.apply

For ES2015, you might want to use Reflect.apply instead.

[see JS: Reflect.apply]

Reference

ECMAScript 2015 §Fundamental Objects#sec-function.prototype.apply